1. Field of the Invention
This invention relates to delayed coking, and more particularly to a method of improving the product yields from a delayed coking operation.
Delayed coking has been practiced for many years. The process broadly involves thermal decomposition of heavy liquid hydrocarbons to produce gas, liquid streams of various boiling ranges, and coke.
Coking of resids from heavy, sour (high sulfur) crude oils is carried out primarily as a means of disposing of low value resids by converting part of the resids to more valuable liquid and gas products. The resulting coke is generally treated as a low value by-product.
In the production of fuel grade delayed coke, and even to some extent in the production of anode or aluminum grade delayed coke, it is desirable to minimize the coke yield, and to maximize the liquids yield, as the liquids are more valuable than the coke. It is also desirable to produce a coke having a volatile matter content of not more than about 15 percent by weight, and preferably in the range of 6 to 12 percent by weight.
The use of heavy crude oils having high metals and sulfur content is increasing in many refineries, and delayed coking operations are of increasing importance to refiners. The increasing concern for minimizing air pollution is a further incentive for treating resids in a delayed coker, as the coker produces gases and liquids having sulfur in a form that can be relatively easily removed.
2. The Prior Art
In the basic delayed coking process as practiced today, fresh feedstock is introduced into the lower part of a coker fractionator and the fractionator bottoms including heavy recycle material and fresh feedstock are heated to coking temperature in a coker furnace. The hot feed then goes to a coke drum maintained at coking conditions of temperature and pressure where the feed decomposes or cracks to form coke and volatile components. The volatile components are recovered as coker vapor and returned to the fractionator. Heavy gas oil from the fractionator is added to the flash zone of the fractionator to condense the heaviest components from the coker vapors. The heaviest fraction of the coke drum vapors could be condensed by other techniques, such as heat exchange, but in commercial operations it is common to contact the incoming vapors with a heavy gas oil in the coker fractionator. Conventional heavy recycle is comprised of condensed coke drum vapors and unflashed heavy gas oil. When the coke drum is full of coke, the feed is switched to another drum, and the full drum is cooled and emptied by conventional methods.
The delayed coking process is discussed in an article by Kasch et al entitled "Delayed Coking," The Oil and Gas Journal, Jan. 2, 1956, pp 89-90.
A delayed coking process for coal tar pitches illustrating use of heavy recycle is shown in U.S. Pat. No. 3,563,884 to Bloomer et al.
A delayed coking process for coal extract using a separate surge tank for the feed to the coker furnace is shown in U.S. Pat. No. 3,379,638 to Bloomer et al.
A process for producing a soft synthetic coal having a volatile matter content of more than 20 percent by weight is described in U.S. Pat. No. 4,036,736 to Ozaki et al. In that reference, a diluent gas is added to the coker drum to maintain a reduced partial pressure of cracked hydrocarbons, or the process is carried out under less than atmospheric pressure.
A discussion of early delayed coking processes appears in an article by Armistead entitled "The Coking of Hydrocarbon Oils," The Oil and Gas Journal, Mar. 16, 1946, pp 103-111.
U.S. Pat. No. 4,216,074 describes a dual coking process for coal liquefaction products wherein condensed liquids from the coke vapor stream and unflashed heavy gas oil are used as recycle liquid to the coker furnace.
U.S. Pat. No. 4,177,133 describes a coking process in which the heavier material from the coke drum vapor line is combined with fresh coker feed as recycle and then passed to a coke drum.
Many additional references, of which U.S. Pat. Nos. 2,380,713; 3,116,231 and 3,472,761 are exemplary, disclose variations and modifications of the basic delayed coking process.
In commonly assigned copending application Ser. No. 464,181, filed Feb. 9, 1983 now U.S. Pat. No. 4,455,219, a delayed coking process is described in which a diluent hydrocarbon having a boiling range lower than the boiling range of heavy recycle is substituted for a part of the heavy recycle that is normally combined with the fresh feed in delayed coking processes.